Utah Code Ann. § 41-6a-605 – Minimum speed regulations.

(1) A person may not operate a motor vehicle at a speed so slow as to impede or block the normal and reasonable movement of traffic except when:
1. (a) a reduced speed is necessary for safe operation;
2. (b) upon a grade; or
3. (c) in compliance with a traffic-control device.
(2) Operating a motor vehicle on a limited access highway at less than the speed limit side by side with and at the same speed as a vehicle operated in the adjacent right lane is evidence of a violation of Subsection (1).
(3)
1. (a) If, based on an engineering and traffic investigation, a highway authority determines that slow speeds on any part of a highway under its jurisdiction consistently impede the normal and reasonable movement of traffic, the highway authority may post a minimum speed limit.
2. (b) If a minimum speed limit is posted under this Subsection (3), a person may not operate a vehicle at a speed below the posted minimum speed limit except:
  1. (i) when necessary for safe operation; or
  2. (ii) in accordance with Section 41-6a-205.
3. (c) The minimum speed limit is effective when appropriate signs giving notice are erected along the highway or section of the highway.